The present invention relates to a matrix type liquid crystal display panel and more particularly to structure of a liquid crystal display panel with high resolution and increase in the number of matrix electrodes.
As a rule, a matrix display is of the type which has orthogonal strip electrodes disposed on it and the portion of the display where the electrodes are crossed forms a picture element thereby providing a visual display of characters, symbols, numerals, patterns of the like in response to selective application of a voltage to the respective X and Y electrodes. The most common problem with the matrix type display is that a voltage may be applied to some extent to a crossing of the X and Y electrodes not desired to operate (termed "non-selected point") in applying a more than threshold voltage to a crossing of the X and Y electrodes desired to operate (termed "selected point"). This causes the crosstalk phenomenon.
Actually in driving address-by-address a matrix type display which takes advantage of electro-optical effects of liquid crystal such as the twisted nematic field effects (TN), the dynamic scattering effects (DSM), the field induced double refraction effects (TB) and the guest host effects (GH), the crosstalk phenomenon often places non-selected points into the operating state, resulting in difficulties in displaying only the desired patterns. This is because the electro-optical effects of liquid crystals have electrically bidirectional features and sometimes show no definite threshold effects. A well known solution to this problem is the voltage amplitude selection method. Typically, an X electrode and a Y electrode are supplied with voltages V.sub.o and O when selected and with voltages 1/3V.sub.o and 2/3V.sub.o when not selected, respectively. As a result, each selected point of the X and Y electrode is supplied with a voltage O and each non-selected point with a voltage 1/3V.sub.o. This is termed the 1:3 voltage average method. In this instance a ratio of effective voltage on the selected point to that on the non-selected point can be represented below: ##EQU1## wherein n is the so-called degree of multiplexing and thus corresponds to the number of scanning electrodes in the XY matric panel.
Analysis of the formula (1) reveals that the ratio of V.sub.s /V.sub.u is reduced with an increase in the number N of the scanning electrodes. V.sub.u is generally selected below a threshold voltage (V.sub.th) of the electro-optical effects of liquid crystal and V.sub.s above the threshold voltage. While taking voltage dependency of the field effects, for example, the TN effects into account, the contrast property is depicted in FIG. 1, with contrast as ordinate and voltage as abscissa. The graph of FIG. 1 shows that contrast is remarkably increased upon a voltage higher than the threshold voltage V.sub.th. The experiments were conducted with 25.degree. C., 1 KHz, sine wave, liquid crystal of type E-8 marketed by B.D.H. Chemicals Ltd., and polarizers of type HN42 marketed by Polaroid Co. The graph is taked along normal direction.
In addition, a display with a reduced value of V.sub.s is rather less attractive from contrast and response standpoints--i.e., poor contrast and slow response, because response characteristics more particularly rising time (.tau..sub.r) is inversely propertional to the square of voltage. The formula (1), therefore, implies impossibility of elevating N beyond a given limit. Though the foregoing has set forth specifically the 1:3 voltage method, it is also possible to generalize the advantages and disadvantages of a matrix type liquid crystal display panel as follows. An X line and a Y line are supplied with V.sub.o and O when selected and with (1/a) V.sub.o and (2/a) V.sub.o when non-selected, respectively. In this case, V.sub.s /V.sub.u can be written below. ##EQU2## Like the formula (1), the above described formula (2) shows the trend for V.sub.s /V.sub.u to reduce with an increased N. As noted earlier, to gain better contrast and quicker response, it is desirable that the number of N be as small as possible. It is however obvious that higher resolution offers a further improvement in the quality of the display in displaying a pattern on a limited area. To this end the number of N should be increased.